1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device constituting a driving circuit using a bipolar transistor with a special structure.
2. Description of the Background Art
A structure of a bipolar transistor which has been known generally is disclosed in "Analysis and Design of Analog Integrated Circuits, Second Edition" by Raul R. Gray and Robert G. Meyer.
FIG. 8A is a plan showing a conventional bipolar transistor described in the above mentioned document, and FIG. 8B is a cross section thereof.
Referring to FIGS. 8A and 8B, in the conventional bipolar transistor, a P type semiconductor substrate 51 is prepared, and a high concentration N type region 52 is formed therein. Then, a low concentration N type region 53 is formed at a main surface of semiconductor substrate 51 by epitaxial growth. Then, a P type base region 59, and high concentration N type regions 60 and 62 which will be emitter and collector regions, respectively, are formed. Thereafter, a metal electrode 63 is connected to each of the emitter, the base and the collector, constituting a bipolar transistor.
FIG. 9B is a diagram showing an impurity concentration profile in the direction of the section A--A' (see FIG. 9A) of the conventional bipolar transistor shown in FIGS. 8A and 8B.
FIG. 9A shows where the section A--A' is located in the bipolar transistor, and FIG. 9B shows an impurity concentration at each location along the section A--A'. An example in which an impurity concentration in collector region 53 formed by epitaxial growth is 1.times.10.sup.15 cm.sup.-3 ! is shown here. High concentration N type region 52 has such a profile as illustrated in the figure.
FIG. 10A is a cross section showing a bipolar transistor having no high concentration buried region, and Fig. 10B is a diagram showing an impurity concentration profile in the direction of the section B--B' of the bipolar transistor as shown in Fig. 10A.
Referring to FIGS. 10A and 10B, a bipolar transistor in this case is formed by diffusion of impurities from a surface of a substrate without using epitaxial growth. Fig. 10B is a diagram showing an impurity concentration corresponding to the depth of each diffusion layer with the abscissa indicating the depth along the section B--B' of the bipolar transistor shown in FIG. 10A and the ordinate indicating an impurity concentration.
Referring to FIG. 10B, the collector has lower concentration than that of a surface (background) of the substrate in a deeper portion of the substrate of this bipolar transistor. Therefore, as described in the above mentioned document (page 86 lines 1 -3), collector resistance is increased and the breakdown voltage between the collector and the emitter is reduced, resulting in the delay of the operation.
On the other hand, in the bipolar transistor having high concentration N type region 52 shown in FIGS. 9 A and 9 B, epitaxial growth of the substrate is required, causing significant increase in cost for the semiconductor substrate. Accordingly, a driving circuit constituted by this bipolar transistor is expensive, resulting in increase in the cost for a semiconductor device including the driving circuit.